A MU (Multi-User) communication is currently proposed as one of technical items under next-generation wireless LAN (Local Area Network) standard. The technical items include UL-OFDMA (Uplink Orthogonal Frequency Division Multiple Access) communication. In UL-OFDMA, a plurality of wireless communication terminals (STA: Station) are allocated to, for example, different frequency subchannels in a frequency channel so as to simultaneously transmit frames.
A frame sequence example of an UL-MU transmission will be discussed below. First, an access point (AP) transmits a trigger frame to specify STAs for permitting the transmission of the frames through a UL. The specified STAs (in this case, STA1 to STA4) transmit the frames (in this case, data frames) to the AP according to a predetermined transmission method such as a frequency subchannel used for transmission and a PHY transmission rate.
The AP having received the frames from the STAs generates a response frame, which includes acknowledgement information on the frames, for the frames received from STA1 to STA4, and then the AP transmits the response frame to STA1 to STA4 after SIFS (Short Interframe Space:=16 μs) from the end of the frame. The response frame may be, for example, a Multi-STA BA (Multi-Station BlockAck) frame.
The Multi-STA BA frame carries the acknowledgement information on the frames received by the AP from the STAs accompanied with STAs' IDs (identifiers) which are locally allocated by the AP to the STAs in order for the STAs to distinguish the corresponding acknowledgement. For the generation of the Multi-STA BA frame, it is required at the AP to have a mechanism to convert the STAs' MAC addresses set in the frames transmitted from the STAs to the STAs' IDs. This is because in the frames received by the AP from the STAs, the MAC addresses are the only fields where the STAs are identifiable at MAC (Medium Access Control) level.
An example of such a mechanism is a method of storing an ID table by the AP, the ID table containing sets of MAC addresses and IDs corresponding to the MAC addresses. In this method, however, the number of STAs which can be connected to the AP is restricted depending on the upper limit amount of information stored in the table. In order to avoid the restriction, the AP needs to provide a memory having a sufficient capacity. Also, the larger the number of STAs connected to the AP, the longer the ID retrieval from the table. As described above, a time period from the reception of the frames from the STAs sent under MU transmission to responding with the Multi-STA BA frame is SIFS. Thus, in order to make a rapid response with a large number of STAs, a hardware resource is more consumed accordingly.